1. Field of the Invention
The present invention relates to an optical fiber applicable to large-capacity, high-speed WDM (Wavelength Division Multiplexing) optical transmission systems; and an optical transmission line including the same.
2. Related Background Art
A WDM optical transmission system enables large-capacity, high-speed optical communications since a plurality of signal lights (hereinafter referred to as WDM signals) in a 1.5-.mu.m wavelength band (1500 nm to 1600 nm) propagate there through by way of a network of optical fiber transmission lines. In general, this optical transmission system comprises an optical amplifier for optically amplifying the WDM signals together, and the like in addition to optical fiber transmission lines which are a transmission medium. In such WDM communications, various techniques are under study in order to enable further larger capacity and higher speed.
How to reduce the dispersion and dispersion slope has been an important subject for study with respect to optical transmission lines. It is because of the fact that, though being monochromatic, each signal light propagating through an optical transmission line has a predetermined bandwidth, whereby the signal light sent out from a transmitting station may deform its waveform when reaching a receiving station by way of the optical transmission line, thus deteriorating its reception.
For securing the quality of an optical transmission line, it is desirable that the dispersion of the optical transmission line in its signal light wavelength band be as small as possible. For realizing larger-capacity communications, on the other hand, it is necessary for the optical transmission line to suppress the dispersion in a wavelength band as wide as possible, and it is desirable that the dispersion slope of the optical transmission line be as small as possible. Therefore, dispersion-flattened optical fibers in which both the dispersion and dispersion slope are substantially zero in the 1.5-.mu.m wavelength band have conventionally been studied for use as an optical transmission line. Here, the dispersion slope refers to the gradient of a graph indicating the wavelength dependence of dispersion.